Field of the Invention
The present invention relates to an image reading apparatus including an illumination device configured to illuminate an original.
Description of the Related Art
Hitherto, an image reading apparatus has been used in a copying machine, an image scanner, a multifunction printer, and the like. The image reading apparatus is configured to read an image of an original by illuminating the original with light and receiving light reflected from the original. The image reading apparatus includes an illumination device configured to illuminate an original with light. There is known an illumination device including a light guide configured to guide light emitted from a plurality of light emitting elements to an original (U.S. Pat. No. 8,876,355).
FIG. 8A and FIG. 8B are views for illustrating a conventional illumination device 550. FIG. 8A is a perspective view of the illumination device 550. FIG. 8B is a cross-sectional view for illustrating a sub-scanning cross-section of the illumination device 550. The illumination device 550 includes a plurality of light emitting elements 100, a circuit board 101 on which the plurality of light emitting elements 100 are arrayed, and a light guide 502 fixed to the circuit board 101. A light beam 107 emitted from each of the plurality of light emitting elements 100 enters an incident surface 503 of the light guide 502. The light beam 107 propagates while repeating total reflection on an inner wall of a light guide portion 504 of the light guide 502. The light beam 107 is deflected by a deflection surface 505 toward an upward travelling direction and emitted from an emission surface 506 as an emission light beam 108. A surface of an original 203 placed on a platen glass 202 is linearly illuminated with the emission light beam 108.
As illustrated in FIG. 8A, the plurality of light emitting elements 100 are linearly arranged on the circuit board 101 at predetermined intervals (distances). FIG. 9A and FIG. 9B are views for illustrating light distribution characteristics of the conventional illumination device 550. FIG. 9A is a graph for showing an illuminance on the original 203 at positions in a main scanning direction. FIG. 9B is a light beam distribution diagram of the conventional light guide 502. The plurality of light emitting elements 100 are arranged so as to be spaced apart from each other, and hence uneven illuminance 113 (hereinafter referred to as “unevenness due to light emitting points”) occurs in the illuminance distribution on the original surface depending on the array intervals of the plurality of light emitting elements 100.
In order to reduce the unevenness 113 due to light emitting points, it is conceivable to form a toric surface array 109 on an emission surface 606 of a light guide 602. FIG. 10A and FIG. 10B are views for illustrating light distribution characteristics of an illumination device 650 including the toric surface array 109 formed on the emission surface 606. The toric surface array 109 comprises a plurality of toric surfaces arrayed side by side in a longitudinal direction of the light guide 602. The toric surface array 109 serves as a secondary light source configured to diffuse the emission light beam 108 emitted from the emission surface 606 in the longitudinal direction of the light guide 602. The toric surface array 109 is configured to reduce the unevenness 113 due to light emitting points as shown in FIG. 9A. However, as shown in FIG. 10A, fine uneven illuminance 114 (hereinafter referred to as “unevenness due to pitch bright points”) occurs depending on the pitch of the plurality of toric surfaces of the toric surface array 109. In particular, there is a problem in that, when reading a glossy original, the unevenness 114 due to pitch bright points occurs on a read image.